Damage To An MCB By Continuous Overcurrent?

[securespark]

I found a shower circuit today, running either (could not ascertain the exact size) a 9.5kW or 10.5kW load on a 32A B type.

What is the likelihood and extent of damage to that breaker?

 

[JohnD]

I have an idea they are designed not to be damaged by an overcurrent which is insufficient to trip the thermal.

may have seen this in an old spec somewhere. might have been Crabtree where they were boasting about a magnetic slug in a viscous liquid to prevent fast trip, y'know what I mean?.

or I could be imagining it.

 

[RF Lighting]

I would think a decent brand of MCB should be able to survive that with out any problem.

I've come across plenty of continuously overloaded MCBs which show no signs of damage, so I'd expect the intermittent loading of a shower to do less damage.

The majority of failed MCBs seems to be from loose / bad connections.

I expect over time the thermal coil jobbie may become damaged making the MCB unable to reset.

I assume you have a photo to prove me wrong?

 

[JohnD]

you want a photo of an MCB that's not damaged

 

[RF Lighting]

I reckon Simon has found a very badly damaged MCB and is just trying to tease us

 

[securespark]

No, not a trick question, Rob!

Just simply that I found this shower circuit today badly overloaded & the user said the CB has never tripped..Just wondered if it hadn't because it was damaged, that's all.

 

[RF Lighting]

I suppose it is quite possible for the MCB to be overloaded, yet still healthy. The intermittent usage of a shower may just fit in nicely with the sustained overload trip characteristics of the MCB.

The shower if rated at 9.5kW and running on a Uoc of 240V it will draw 39.5A. According to my tripping curve graphs, a 32A type B MCB will hold this overload indefinatly, and will not start to trip until ~42A, and even this requires the overload to be sustained for over 30 mins, which is more than enough time to have a shower and switch it off. This then gives the thermal coil time to cool down before the shower is next used.

 

[Adam_151]

As rob says, it doesn't necessaraly follow that the MCB was defective becuase it failed to trip with that load, there is a figure of 1.45xIn* which requires the breaker to trip within reasonable time (30mins?), and I beleive BSEN60898 also defines a lower limit of 1.15In to reduce nuisense, which works out at between 36.8A and 46.4A, but I'd imagine most modern breakers would end up near the more senstive end of the scale.

I think generally breakers that have been weakend by overcurrent are likely to weaken in the way that that'll start tripping easier, rather than vice versa

GN3 does suggest that as there is no way to field test overcurrent circuit breakers, that if there are concerns about their reliability that they are repaced

* - (The 1.45 figure, I beleive is derived from it being the root of two...increase I by 1.45 and you double I², thus double power dissipated in a resistance, thus double the temperature rise from 40 to 80, so instead of running at 30+40=70degrees, the cable runs at 30+80=110degrees, which is the temperature above which the cable stops merely aging faster and the conductors begin to migrate through the PVC, theres a bit of a bang, power goes off, sparky is duly called, and householder claims he knows sweet FA about how a hairpin ended up in a BS3036 carrier...)